Selective Spraying

Conformal coating selective robot spray

Co-Authors and Contributors

  • Dr Lee Hitchens, Nexus
  • Kara DeConno, PVA
  • Gordon Watson, SMS Automation

https://player.vimeo.com/video/172928820

Selective spraying using an automated robot system is one of the widest used methods in conformal coating application and processing.

The conformal coating is applied selectively to the circuit board to the areas requiring coating only.

The selective process deliberately does not apply the coating to areas that normally require masking such as connectors and other components.

This process of selectively applying the conformal coating to the circuit board can avoid using a time consuming masking process and costs can normally be reduced.

Read more about the various conformal coating spray valves here.


The key areas for selective robotic spraying

  1. What equipment do you need for selective spraying?
  2. What are the specialist valves required?
  3. Which conformal coatings can be used in selective spraying?
  4. Is selective spraying of conformal coating a complex process?
  5. How cost effective is selective spraying of conformal coating as a process?
  6. Is selective spraying of conformal coating a low, medium or high-volume technique?
  7. What variables control the quality of the conformal coating finish in selective spraying?
  8. What are five advantages of the selective spray conformal coating process?
  9. What are five disadvantages of the selective spray conformal coating process?

Read more below to find each of these questions answered. 


What equipment do you need for selective spraying?

A specialist robotic system designed for application of conformal coatings is normally required for selective coating.

This system is a purpose-built cartesian robot fitted with  various conformal coating spray valves that apply the material to the circuit board using different spray patterns.

The level of movement of the robot can vary with systems having three, four, five and even six axes of movement for the spray valves.

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A specialist robotic system designed for application of conformal coatings is normally required for selective coating.

What are the specialist conformal coating valves required?

What is critical for a successful selective conformal coating application process are the various specialist conformal coating spray valve technologies the robot must use.

These valves must be suitable to use with the conformal coating material selected.

Read more about the various conformal coating spray valves here.

They must also be capable of applying the coating correctly to the areas of the board required to be coated.

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This combination of valve selection, the conformal coating material type and the configuration of the circuit board is the most critical factor in obtaining the optimum performance from the selective coating system.

Selecting the wrong individual valve or the wrong combination of 2-3 valves for the conformal coating process can make a project costs increase sharply.

For example, some conformal coatings are not compatible with certain spray valve technologies and care has to be taken in matching the material to the valves.

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Needle dispensing a conformal coating can give precision application to difficult areas and can be used in combination with other wider area coating valves. However, it would not be suitable for large area coverage.

Read more about the various conformal coating spray valves here.


Which conformal coatings can be used in selective spraying?

All conformal coatings can normally be applied by selective robotic spraying.

However, the material may be needed to be matched to the correct conformal coating spray valves and the conformal coating viscosity may need to be adjusted to allow the coating to spray correctly.

Other considerations are the pot life of the material and whether the conformal coating changes its properties whilst in the process of application.

This is significant when using moisture cure and two part coatings.

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The atomised spray valve can apply almost all commercially available conformal coatings to printed circuit boards

Is selective spraying of conformal coating a complex process?

The programming of a robotic system can be a very skilled process.

The software can make it easier but generally it is a technical process.

The operator programming the machine must:

  • Determine which areas are to be coated (may be decided at the conformal coating design stage)
  • Identify which spray valves will work best (working with what he has available) for applying the conformal coating material selected
  • The spray pattern must be developed to apply all the coating correctly to the different areas of the circuit board without applying the material in the wrong areas or applying too little or too much coating
  • Consider the material as a liquid that will flow on the circuit board after application and avoid conformal coating defects like capillary flow
  • The pattern must be efficient and avoid damaging the board

This requires lots of training, a development of knowledge and a build up of experience to achieve an acceptable process on a regular basis.

Once programmed the conformal coating selective system now has to be operated in production.

Again, the machine is a complex robotic system with many different variables.

However, the recent developments in modern programming software should help with the operation.

For the operation there are several factors to consider including:

  • Material performance in the machine over time (the material can change characteristics)
  • Manage stoppages due to blockages in coating, misfires and other normal process problems
  • Control any changes in the environment that may affect the conformal coating application (for example, several degrees of temperature change can effect conformal coating application)
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The combination of programming competence required by the engineers and the skill level needed for operation means that a selective robotic conformal coating process is the most demanding of all the application processes for achieving a repeatable, high quality conformal coating finish.

How cost effective is selective spraying of conformal coating as a process?

Selective spraying is a not a low cost set up.

It normally requires considerable capital investment in each system

It also takes a lot of training of staff and a lot of time to set up correctly to ensure long term success.

However, the selective robotic system can handle highs volume of product so the Return on Investment (ROI) can be extremely good over time.

For high volume processing, where the elimination of masking reduces the costs of the process, then this is an extremely cost-effective process.

For lower volumes, the costs need to be balanced against the quality of coating and the masking time saved.


Is selective spraying of conformal coating a low, medium or high-volume technique?

Normally selective robotic coating is a medium or high volume process.

It can be used in low volume but it can be very inefficient in costs compared to a simple manual process.

In low volume processing, the selective systems are normally used to apply repeatable coating coverage where cost is not a consideration but consistency is critical.


What variables control the quality of the conformal coating finish in selective spraying?

Variables that influence the quality of the conformal coating process in selective spraying are:

The operator skill

The programming and control of the robotic system and the material variables require a high level of skill and knowledge to master.

Losing the operator skill is one of the number one problems with conformal coating robots.

Quality of the selective spray equipment

The different robotic systems have different technologies, valve configuration, options and build qualities.

There is a case here for the more you pay, the more likely you will achieve a repeatable, high-quality finish in the case of the coating robots.

Valve technologies

The match of the valve technology to the conformal coating material and the circuit board is a crucial decision that can influence costs, quality of finish and process control.

Viscosity of the coating

Blending the conformal coating to the right viscosity is critical.

The wrong conformal coating viscosity can produce many conformal coating defects and also give process problems.

Consider using pre blended coatings to remove one variable from the process.

Circuit Board Design

Some circuits are just not suited to selective robotic coating.

Others require the valve technologies being matched.

Coating deposition requirements

The coating coverage repeatability is limited by the robot’s accuracy, the material characteristics and the board design.

Care should be taken in specifying coating thickness and coverage tolerances that are not realistic.

Ambient Conditions

The temperature and humidity of the surrounding area of the robot can influence the quality of the finish.


What are five advantages of the selective spray conformal coating process?

  1. Masking can be eliminated or minimised
  2. High speed coating process
  3. High volume process
  4. High quality finish when under control
  5. Simple process to operate when set up

What are five disadvantages of the selective spray conformal coating process?

  1. High engineering skill required for set up and programming
  2. High engineering skill required for troubleshooting and supervision
  3. High capital cost initially
  4. Process controls need to be enforced for good quality production
  5. Environment can influence the process controls
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